The present invention relates to the inventions set forth in commonly assigned U.S. Pat. No. 6,218,680 (“the '680 patent”); U.S. Pat. Nos.6,396,080; 6,403,982; 6,639,247; and 6,507,046.
The present invention relates to semi-insulating silicon carbide single crystals, and in particular, relates to a method of forming high purity semi-insulating silicon carbide single crystal substrates that have intrinsic point defects and resulting deep level electronic states in an amount greater than the net concentration of any compensating shallow dopants (i.e., an amount greater than the uncompensated shallow dopants), and to maintain the semi-insulating quality of the silicon carbide substrate during additional process steps of device manufacture.
As set forth in the '680 and related patents, it has been discovered that semi-insulating silicon carbide can be produced without the use of vanadium as the dopant to create deep level states that produce the semi-insulating character.
Although vanadium can produce a semi-insulating silicon carbide crystal, its presence has been observed to create a back-gating effect; i.e., the trapped negative charge on the vanadium acts as a grown-in gate in devices in which a vanadium-doped crystal is used as the semi-insulating substrate. Thus, for a number of device considerations, vanadium is best avoided.
In the '680 patent and the related applications, a semi-insulating silicon carbide crystal is described that includes donor dopants, acceptor dopants and intrinsic point defects that produce deep level states. When the concentration of intrinsic point defects exceeds the difference between the concentration of donors and the concentration of acceptors, the states resulting from intrinsic point defects can provide semi-insulating characteristics in the functional absence of vanadium; i.e., including a minimal presence that is less than the presence that can affect the electronic properties of the crystal.
The requirements for and the advantages of semi-insulating substrates, their use in devices, particularly microwave devices, and the associated and particular requirements for silicon carbide semi-insulating substrates are set forth in detail in the '680 patent and related applications, and are generally well understood in the art from a background standpoint. Thus, they will not be repeated in detail herein. For reference purposes, a relevant discussion is set forth in the '680 patent at column 1, line 14 through column 3, line 33, which is incorporated entirely herein by reference.
To this discussion it should be added, however, that the ever-increasing demand for wireless communication services, including high bandwidth delivery of Internet access and related services, drives a corresponding demand for devices and circuits that can support such delivery, which in turn calls for materials—such as semi-insulating silicon carbide—from which devices having the required capabilities can be manufactured.
Accordingly, the '680 patent explains that superior microwave performance can be achieved by the fabrication of silicon carbide field effect transistors (FETs) and related devices on high purity, vanadium-free semi-insulating monocrystalline silicon carbide substrates. As set forth in the '680 patent, the substrates derive their semi-insulating properties from the presence of intrinsic (point defect related) deep electronic states lying near the middle of the silicon carbide bandgap. The intrinsic deep states generally arise during growth of a crystal boule at high temperatures from which substrate wafers are cut in a manner generally well understood in this art.
In devices that incorporate these substrates, and in order to provide the appropriate low-loss RF performance, the substrate must act as a low-loss dielectric medium by continuously maintaining its semi-insulating characteristics. In turn, the ability to maintain semi-insulating behavior is dependent upon the total number of intrinsic deep states in the substrate. In current practice, if the density of the intrinsic deep levels is not sufficiently high it has been observed in practice that the semi-insulating characteristics of the substrate can become reduced or functionally eliminated when subsequent steps are carried out on or using a semi-insulating silicon carbide wafer. Such steps include the growth of epitaxial layers at temperatures of about (for illustrative purposes) 1400° or above on the semi-insulating silicon carbide wafer. This in turn reduces the number of useful devices that can be formed on or incorporating the wafers.
Although the inventors do not wish to be bound by any particular theory, it appears that when semi-insulating silicon carbide substrate wafers of this type are subjected to process steps at temperatures within certain ranges, the subsequent processing can act as an anneal that reduces the number of point defects. This can be thought of in the positive sense that a higher quality crystal is created, but it is disadvantageous when the number of intrinsic point defects is the basis for the semi-insulating character of the substrate wafer.
Stated differently, if kept within a particular temperature range for a sufficient time, the crystal equilibrium or near-equilibrium will shift to one in which the number of point defects is reduced; i.e., the crystal becomes more ordered (fewer point defects) at lower temperatures than it was at higher temperatures, in a manner expected in accordance with well-understood thermodynamic principles.
Accordingly, a need exists for silicon carbide substrate wafers that incorporate the advantages set forth in the '680 patent, and that can maintain these advantages during subsequent manufacture of devices and circuits on or incorporating the semi-insulating silicon carbide substrate wafers.